In order to obtain such support, it is necessary, in theory, to cause an alternating magnetic field to pass through a part which, in order to obtain the best possible results, should be as nonmagnetic as possible and as good a conductor of electricity as possible. This part is often in the form of a horizontal plate, or sheet or the like. At the part, said field has two perpendicular components:
one of which is vertical (parallel to gravity and perpendicular to the part) and may be called the "through" component of the field; and
the other of which is horizontal and may be called "tangential" component since it runs parallel to the part.
The flux of the vertical or through component induces electrical currents in the part. By application of Fleming's rule, these induced currents co-operate with the tangential component of the field to create vertical forces. If the geometry of the system is suitable, this force opposes gravity and "levitates" the part. FIG. 1 is an elevation view of a prior art device applying this principle to a relatively thin metal sheet. The figure shows:
the sheet 1 to be levitated which extends horizontally between two networks of magnetic field generator devices ("poles");
a bottom network 2 of poles which alternate between north polarity (marked N) and south polarity (marked S); and
a top network 3 disposed symmetrically to the preceding network with same-polarity poles facing one another and with the resulting magnetic field being represented by arrows H.
In theory, it is not essential to use two networks of poles, nor is it essential (when two networks are used) for facing poles to be of the same polarity, but this is the disposition which provides best device efficiency. If facing poles are of opposite polarity, then there is too much through flux and this gives to rise to undesirable heating due to the Joule effect in the levitated part.
FIG. 2 shows one element (or "mesh") forming part of a plate to be levitated between two facing poles, and correpsonding to said pair of poles. This element is in the form of a quadrilateral Q. It may, for example, be square and form a part of a regular two dimensional network extending over the entire area of the sheet and resulting from the corresponding regular disposition of the north and south poles. The through component of the field Hv induces a current i at a point P where the tangential component of the field is Hh. Said component and said current create a vertical levitation force FS in accordance with Fleming's rule which is applied at point P. If the sheet is equidistant between the two identical networks of poles, the through component Hv is zero (by symmetry) and so the force FS is also zero. The sheet therefore moves towards the bottom network until the levitation forces are such that FS becomes large enough to support the sheet.
In currently produced devices, the poles are formed by windings having an excitation current flowing therethrough and optionally being wound around a laminated magnetic circuit: the term "winding" is used herein to designate the well-known device constituted by a relatively fine electrical conductor wound in a greater or lesser number of coaxial loops or "turns".
The poles are disposed in a manner which matches the shape of the parts to be levitated. In order to levitate a plane horizontal sheet, the poles are distributed above and below the sheet as shown in FIG. 3. The windings of the bottom network 2 are referenced NB and SB and the windings of the top network 3 are referenced NH and SH.
Connecting wires must be placed between the windings: the windings NB and SB in a given row are, for example, connected in series by wires such as 5 and 7 belonging to said row and themselves connected to wires such as 9 and 11 which are common to all of the rows of a given network.
These devices using windings suffer, in practice, from drawbacks due to the volume they occupy in the plane in which they are laid, and from their fragility in the face of the attacks to which they are exposed from various different sources:
In particular there are the following drawbacks:
(1) the levitation area above and below the sheet is largely occupied by windings and their connection wires, thereby limiting or preventing useful devices from occupying said area (e.g. heating or cooling devices, devices for performing inspection or measurement . . . ). This is referred to by saying that the device is not sufficiently transparent; and
(2) the windings and the connection wires are necessarily made from wires which are insulated using an insulating varnish or enamel, since these wires touch one another and cross at numerous points. They therefore constitute relatively complex and fragile assemblies, in particular in the presence of hot sheets.
The present invention seeks to provide magnetic levitation while eliminating these drawbacks by providing a device which is both transparent and robust. Beginning with the general, the subject matter of the present invention may be defined by the following points: